This invention relates to an apparatus applicable to video tape recorders and the like for providing accurate servo control of motor rotation.
Motor speed servo control apparatus have been employed for controlling the speed of rotation of motors used to drive VTR drums. It is the current practice to perform such motor speed control based upon a motor speed signal having a series of FG pulses generated at a frequency corresponding to the motor speed sensed by a frequency generator or the like sensor operable in connection with the motor. One of serious problems is reproduced picture distortion caused by noise superimposed on the motor speed signal when the drum or the frequency generator is mounted in an erroneous manner to cause drum rotation variances. Such noise superimposed on the motor speed signal has a frequency, for example, 30 hertz, corresponding to the frequency of rotation of the drum.
In order to avoid such a problem, it is the current practice to design the motor speed servo control apparatus to have a characteristic reducing the gain at a frequency around 30 hertz. However, such gain reduction raises another problem in that the servo control accuracy is influenced considerably by torque variations.
Alternatively, it has been proposed to provide an analog notch filter to attenuate the gain at a frequency around 30 hertz. However, this proposal is not suitable for applications to video tape recorders or the like where the speed of rotation of the drum is changed to accommodate different modes of operation of the apparatus so that the frequency of the noise superimposed on the motor speed signal due to drum rotation variances changes from 30 hertz. Therefore, it is not appropriate to minimize reproduced picture distortion by employing a filter having a high Q at a frequency around 30 Hz.